Widely used quartz crystal based resonators can potentially be replaced by micromechanical, typically silicon-based resonators in many applications. Silicon resonators can be made smaller than quartz resonators and there are a plurality standard manufacturing methods for silicon resonators. However, a problem associated with silicon based resonators is that they have a high temperature drift of the resonance frequency. The drift is mainly due to the temperature dependence of the Young modulus of silicon, which causes a temperature coefficient of frequency (TCF) approx. −30 ppm/° C. This causes the resonance frequency to fluctuate due to changes in ambient temperature.
The large intrinsic temperature drift prevents silicon-based resonators from entering the quartz crystal dominated oscillator market. It is, however, known to compensate the temperature dependence in various ways. The prior art solutions include:                Active compensation with a temperature sensor and related electronic control circuitry, but it has not been possible to provide a resonator with sufficiently low temperature drift with low cost technology which would be suitable for mass production applications and would compete with quartz quality. Also, the use of a temperature compensation circuit increases the consumption of energy, which is a significant disadvantage especially in battery operated devices. Further, the compensation circuit tends to increase electric noise in the resonator circuit.        Active compensation by stabilizing the temperature of the resonator with temperature isolation and controlled warming/cooling of the resonator. However, this solution also increases the energy consumption of the device, and makes the device complicated to produce. The temperature compensation circuits are also slow in controlling, and cannot therefore compensate fast or large changes in ambient temperature sufficiently well.        Passive compensation by addition of amorphous SiO2 exhibiting opposite sign of temperature drift to the structure. This, however, leads to a more complex fabrication process and resonator performance trade-off        Passive compensation by heavy p-type doping, such as boron doping, compensates strongly for c44 characterized shear modes, like the Lamé mode, but less or not at all some other modes, limiting the applicability to quite special modes and excitation geometries in the case of piezoactuation. In particular, extensional modes are not well compensated by p-type doping.        
Passive compensation methods are discussed in not yet published Finnish patent applications 20105849 and 20105851 of the same applicant, as well as references cited therein, in particular A. K. Samarao et al, “Passive TCF Compensation in High Q Silicon Micromechanical Resonators,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2010), 2010, pp. 116-119; US 2010/0127596 and U.S. Pat. No. 4,719,383.
An article by A. K. Samarao et al., “Intrinsic Temperature Compensation of Highly Resistive High-Q Silicon Microresonators via Charge Carrier Depletion”, Frequency Control Symposium (FCS), 2010 IEEE International, 1-4 Jun. 2010, pages: 334-339, discloses a bulk acoustic wave resonator comprising boron-doped (p-doping) silicon resonator element for reducing the temperature drift of the resonator. The boron-doped (p-doping) resonator element comprises, in addition, one or more layers of diffused phosphorus (n-doping) in order to create one or more pn-junction to the resonator element. The pn-junctions forming a depletion region with low charge carrier concentration have the effect that a TCF of −3 ppm/° C. could be achieved.
Hajjam et al. “Sub-100 ppb/° C. Temperature Stability in Thermally Actuated High Frequency Silicon Resonators via Degenerate Phosphorous Doping and Bias Current Optimization”, IEEE International Electron Device Meeting, December 2010, disclose also the possibility of n-type doping of silicon with phosphorus in order to further improve the TCF. They report a temperature drift of 0.05 ppm/° C. in a thermally diffusion doped silicon resonator. Diffusion doping, however, results in a strong concentration gradient of charge carriers in the resonator and an n-dopant concentration of about 1*1019 cm−3 or higher, which is shown later to be needed for efficient T-compensation, can be created only to a region penetrating to approximately 2 micron thickness from the surface of the device. Achieved concentration levels may be also dependent on the exact geometry of the device, which sets design constraints. Thus, there are severe limits for the design of the resonator with respect to its volume, thickness and availability of resonance modes, for example. For example, bulk acoustic wave modes are not effectively temperature compensated in diffusion doped resonators.
U.S. Pat. No. 4,358,745 discloses a surface acoustic wave (SAW) device having a substrate comprising a thin doped silicon layer carrying surface waves and being allegedly temperature compensated. Modern simulations have, however, shown that the structures described therein can carry only Rayleigh SAW waves and shear horizontal SAW waves which are not well temperature compensated in practice due to strong contribution from non-compensated elastic matrix elements of silicon. In addition, the publication does not disclose any excitation means which could in practice be used to excite a SAW mode to the structure disclosed. Introduction of such means, such as a piezoactive layer on top of the doped layer, would additionally decrease the performance of the device since the comtribution from the piezoactive layer would be very large. Due to these facts, the described structure has never been commercially exploited.
Thus, there is a need for improved and practically feasible semiconductor resonators and other devices.